1. Field of the Invention
The present invention relates to a multiphase DC-DC converter, and more particularly, to a useful technique which is applied to a multiphase DC-DC converter using a PFM (Pulse Frequency Modulation)-controlled DC-DC converter circuit.
2. Description of Related Art
There has been known a DC-DC converter of a multiphase system in which a plurality of DC-DC converter circuits are arranged in parallel and operated so that output phases of the respective DC-DC converter circuits are shifted to each other, and the respective outputs thereof are added up to obtain one output. According to such a multiphase DC-DC converter, a large output having a low ripple as a whole can be obtained.
Japanese Patent Application Laid-Open Publication No. 2003-284333 discloses a DC-DC converter of a multiphase system in which PWM (Pulse Width Modulation)-controlled DC-DC converter circuits are arranged in parallel. Japanese Patent Application Laid-Open Publication No. H11-127573 and Japanese Patent Application Laid-Open Publication No. H8-84465 disclose a power supply circuit in which a plurality of DC-DC converter circuits are arranged in parallel although they are not based on the multiphase control system.
In the multiphase system, since the respective DC-DC converter circuits are operated so that the output phases thereof are shifted to each other, the multiphase system can easily be applicable to PWM (Pulse Width Modulation)-controlled DC-DC converter circuits, but it is difficult to apply the multiphase system to PFM (Pulse Frequency Modulation)-controlled DC-DC converter circuits.
To put it concretely, the PWM control is compatible with the multiphase system by changing the pulse width of a driving pulse without changing a period of the driving pulse in order to vary the outputs of the DC-DC converter circuits as shown in FIG. 12A. Accordingly, since a plurality of DC-DC converter circuits are respectively operated at different predetermined operation timing while shifting the operation timing by a predetermined phase angle, it is possible to adjust the outputs of the plurality of DC-DC converter circuits individually.
On the other hand, under the PFM control of FIG. 12B, there is a need to change frequency of the driving pulse in order to vary the outputs of the DC-DC converter circuits. Therefore, when adjusting outputs of a plurality of DC-DC converter circuits individually, the PFM control is not compatible with an operation of the multiphase system in which the plurality of DC-DC converter circuits are respectively operated at different predetermined operation timing while shifting the operation timing by a predetermined phase angle.
Even in the case of PFM control, a plurality of DC-DC converter circuits may have the same configuration as each other by using identical circuit elements for all of the converter circuits so that equal outputs are obtained from the respective converter circuits when operating the circuits at the same frequency. Such an ideal operation, however, cannot be obtained in the actual circuits. That is, since there is characteristic dispersion among the respective circuit elements, the output voltages of the respective converter circuits would be different from one another if they are operated at the same operation frequency. Furthermore, in the operation of the multiphase system, because the outputs of the respective converter circuits are directly connected to one another, slight differences among the output voltages can be appeared as large differences in output currents.
For example, as shown in FIG. 13, when a current of 100A is outputted by adding up the outputs of four DC-DC converter circuits, slight characteristic dispersion of the circuit elements appears as large output dispersion such as 10A to 40A unless the outputs of the respective DC-DC converter circuits are adjusted individually. Furthermore, there may occur such a situation that an output of one DC-DC converter circuit is excessively large while a current of another DC-DC converter circuit flows back.